Colours in Drawing and Graphics Contexts

All colours are specified in terms of a 24-bit colour value, represented by the class TRgb. This 24 bit value is mapped down to device-level values by the graphics device or printer driver.

Note that:

Colours can be specified logically (for example the window background colour), as well as physically. The UI Graphics Utilities API contains a list of logical colours (TLogicalColor) and provides the means to convert them into RGB values.
The window server defines display modes that represent colour resolutions. In each display mode, as enumerated by TDisplayMode, a unique index can represent each physical colour supported. TRgb can map such indices onto a full RGB value.